The present invention relates to a tool posture control system, and more particularly to a system for controlling the posture of a tool while searching for an arc starting point by controlling the posture vector of the searching posture of the tool using a tool coordinate system, and by tilting the tool in a certain direction in a searching coordinate space for thereby seaching for the arc starting point.
Industrial robots have found widespread use and are being used in many fields in recent years. In welding technology, welding operations are getting too difficult for human beings to perform, and the working environments such as for automobile production lines are more likely to be hard on workers. For these reasons, human operations are being replaced with welding robots.
FIG. 4 of the accompanying drawings shows a general welding robot. The illustrated welding robot is an articulated robot having six axes. These six axes include a T(.theta.)-axis about which an arm assembly rotates, a W-axis about which a lower arm is tilted back and forth, a U-axis about which an upper arm is tilted vertically, an A-axis about which a wrist rotates in a horizontal plane, a B-axis about which the wrist moves in a vertical plane, and a C(r)-axis about which the wrist rolls, these axes being independently controlled. Designated at 1 in FIG. 4 is a base on which the articulated robot is supported. A T(.theta.)-axis servo motor 2 is mounted in the base 1 for turning the axes about the vertical axis (Z-axis). On the T(.theta.)-axis servo motor 2, there is mounted a T(.theta.)-axis unit 3 rotated by the servo motor 2. A W-axis unit 4 is fixedly mounted on the T(.theta.)-axis unit 3, and a W-axis arm 5 is rotatably supported by a pivot shaft 5a on the W-axis unit 4, the W-axis arm 5 being operated by a W-axis drive mechanism 6. A U-axis arm 7 is rotatably supported by a pivot shaft 7a on the distal end of the W-axis arm 5, the U-axis arm 7 being operated by a U-axis drive mechanism 8. A wrist mechanism 9 is mounted on the distal end of the U-axis arm 7. The wrist mechanism 9 is rotated by an A-axis servo motor 10, vertically swung by a B-axis servo motor 11, and rolled by a C-axis servo motor 12. Robot operation is performed by a tool attached to the wrist mechanism 9. A torch used as such a tool and an arc welding process employing the torch will be described. FIG. 5 schematically shows an arc welding machine. A wire WR is fed by rollers FR in small increments in the direction of the arrow, and passes through a guide member GB to project from the distal end of a torch TC. The rate of feed of the wire WR is limited such that the distal end of the wire will be spaced a prescribed distance from the surface of a member WK to be welded. The positive potential of a high voltage which is generated by a welding power supply PS intermittently with a given period is applied to the wire WR through the guide member GB, whereas a negative potential is impressed on the member WK to be welded. A gas is supplied from a gas supply (not shown) in the direction of the arrows through the torch TC and applied to the member WK to prevent a welded area from being oxidized. When the gas is supplied from the gas supply and the high voltage is intermittently generated by the welding power supply PS while the wire is fed out in small increments, an arc is produced from the distal end of the wire, and the wire and the member to be welded are melted such that the melted portion is integrally welded. Such a welding operation is performed by the robot. More specifically, the torch of the welding machine is gripped by the robot, and the torch (distal end of the wire) is moved by the robot along a welding path to weld the desired portion.
When the member to be welded is set in place for the welding operation, the torch is moved with respect to the member to be welded to search for an arc starting point while manually searching for the position of the member to be welded.
For determining the tool position, there are employed a robot reference coordinate system, a robot hand coordinate system, and a tool coordinate system. FIG. 6 is a diagram explanatory of such robot, hand, and tool coordinate systems. The robot reference coordinate system is indicated by x, y, z with O denoting the origin. Designated at l, m, n are hand posture vectors in the robot hand coordinate system, while designated at t, u, v are tool posture vectors in the tool coordinate system. Denoted at TCP is a tool center point (also referred to as a tool grip point).
Prior to starting arc welding, as described above, it is necessary to search for an arc-welding starting point. The arc starting point cannot accurately be spotted and the desired operation to search for the arc starting point cannot be effected unless the posture of the torch is appropriately controlled.